ARP

What is ARP and how is it work ?

ARP :

As we know in OSI model when data need to send to network its necessary to encapsulated in frames to passing over network .

In layer 4 TCP/UDP headers adds on it and Segment was created . then it passed to layer 3 to add source and destination IP address and Packet is made .after , its need to encapsulate to frame and add Ethernet header and trailer on packet .in this step Frame is made.

you know for sending data , network protocol need source and destination IP address with source and destination mac-address . usually source and destination IP address are define , additionally source Mac-address is define too . so we need know destination mac-address ? ARP is a layer 3 protocol could find mac-address based on knowing IP address .
ARP request is a broadcast request that sends its request to all of hosts in the network with specified IP address to fined specified mac-address .
from example like below figure if HOST A want to send data to host B ,host A need to know host B IP address and mac-address to . if host A doesn't know(host A first check its ARP table ) host B IP address sends a request to host B and ask it's IP address . (DNS is a protocol that find IP-address based on name) .When host A find host B IP address its necessary to know host B mac-address too . so sends a request to host B to find its mac-address . and ARP do this step .
ARP results saves in a table called ARP cash . ARP cash include IP-address and mac-address of each device(host).this is a ARP cash :

X:\>arp -a

Interface: 192.168.0.31 --- 0x2
Internet Address Physical Address Type
192.168.0.1 00-30-48-53-71-51 dynamic
192.168.0.3 00-30-48-2a-75-bc dynamic
192.168.0.4 00-30-48-80-60-4a dynamic
192.168.0.98 00-0c-f1-6c-55-c7 dynamic
192.168.0.108 00-18-37-05-5b-07 dynamic
192.168.0.121 00-1a-80-4a-5a-cb dynamic
192.168.0.127 00-13-e8-da-d8-0b dynamic

• use arp -a on your windows base PC or show ip arp cammand on your router to see ARP cash.

• ARP protocol for each ARP request first check ARP cash and if cant find any thing , then send the request with source and destination mac-address with : 00:00:00:00:00:00 (6 bytes) and Ethernet destination field is : ff:ff:ff:ff:ff:ff (6 bytes) for broadcasting.

• If destination host is not on a same subnet with source host then the ARP request send to default gateway .with source mac-address and default gateway mac-address for destination field and destination IP address .
• hosts need to use ARP only once in a while (why ?)
  

Proxy ARP :
Can help hosts on a subnet to reach remote subnets without configuring routing or a default Gateway.
Proxy Arp is a kind of ARP that runs on router .
imagine in your network you have more than 1 network IP-address so you should use a router to route your packets and data to each other . .if your router is fail down what happens ? your requests to another network can't reach and cause a problem. if you run Proxy ARP on your router , the router pass arp-cash to all over the hosts in a network and each host can find many mac-address of the other devices as soon as possible in it's own arp-cash.

• using Proxy ARP will definitely increase the amount of traffic on your network segment , and hosts will have a larger ARP table used to handle all the ip-to-mac address mapping .
• Proxy ARP is configured on all Cisco routers.
  
• You can instead HSRP (Hot standby router Protocol) with Proxy ARP .
• Proxy ARP isn't realy separet protocol. its a service run by routers on behalf of other device such as PCs that are separated from others by a router.
  

IARP : (Inverse ARP) works like ARP but it maps the DLCI to IP address in a frame-relay

AARP : (Appletalk ARP) . the protocol in an appletalk that maps datalink adderss to network address.